Aluminum-silicon alloys containing less than about 11.6% by weight of silicon are referred to as hypoeutectic alloys, while alloys containing more than 11.6% silicon are referred to as hypereutectic alloys.
Hypoeutectic aluminum-silicon alloys have a microstructure consisting of primary aluminum dendrites with a eutectic composed of acicular silicon in an aluminum matrix. On the other hand, hypereutectic aluminum-silicon alloys, those containing more than 11.6% silicon, contain primary silicon crystals which are precipitated as the alloy is cooled from solution temperature. Due to the large precipitated primary silicon crystals, these alloys have good wear resistant properties, but are difficult to machine, a condition which limits their use as casting alloys. While alloys of this type have good fluidity, they have a large or wide solidification range, and the solidification range will increase dramatically as the silicon content is increased.
Normally a solid phase in a "liquid plus solid" field has either a lower or higher density than the liquid phase, but almost never the same density. If the solid phase is less dense than the liquid phase, floatation of the solid phase will result. On the other hand, if the solid phase is more dense, a settling of the solid phase will occur. In either case, an increase or widened solidification range will increase the time period for solidification and accentuate the phase separation. With a hypereutectic aluminum-silicon alloy, the silicon particles have a lesser density than the liquid phase, so that the floatation condition prevails and the alloy solidifies with a large mushy zone because of its high thermal conductivity and the absence of skin formation typical of steel castings. As the solidification range is widened the tendency for floatation of large primary silicon particles increases, thus resulting in a less uniform distribution of large silicon particles in the cast alloy.
Hypereutectic aluminum-silicon alloys containing precipitated primary silicon crystals have had commercial applicability only because of their refinement of the primary silicon phase by phosphorous additions to the melt, as disclosed in U.S. Pat. No. 1,387,900. The addition of small amounts of phosphorous causes a precipitation of aluminum-phosphorous particles which serve as the active nucleant for the primary silicon phase. Due to the phosphorous refinement, the primary silicon particles are of a smaller size and have a more uniform distribution, so that the alloys can be used in applications requiring the manufacturing attribute of machinability, and the engineering attribute of wear resistance.
It has been found that if an engine block for an internal combustion engine, as well as the pistons, are both formed of a hypereutectic aluminum-silicon alloy, "pull out" damage and sub-surface cracking damage can occur at the mating surface interface, as the primary silicon particles in one of the mating surfaces contacts and attempts to dislodge the primary silicon particles in the other mating surface. To avoid this problem in the past, a harder metal, such as chromium or iron, has been plated on one, but not both of the mating surfaces. For example, in marine engines it has been proposed to plate the cylinder bores of a hypereutectic aluminum-silicon alloy engine block with chromium and utilize pistons of an unplated hypereutectic aluminum silicon alloy. It has also been known to utilize chromium plated pistons with linerless unplated hypereutectic aluminum silicon engine blocks. However, both of these systems require expensive chromium plating on one of the components to avoid the wear damage mentioned above.
In high performance racing engines, it has also been proposed to coat the cylinder bores of a hypereutectic aluminum silicon engine block with electroplated nickel and silicon carbide and utilize uncoated hypereutectic aluminum-silicon alloy pistons with this block. This combination has shown to be workable, because the silicon carbide particle size of the cylinder bore coating is much smaller than the primary silicon particle size of the aluminum-silicon alloy pistons, and because the hardness of the electroplated nickel is significantly greater than the hardness of the aluminum-alloy matrix of the hypereutectic aluminum-silicon alloy. In effect, the large primary silicon particles of the piston alloy do not dislodge the smaller silicon carbide particles because the hard nickel matrix resists the furrowing tendencies of the primary silicon particles.
The commercial problem with any of the above-mentioned piston and cylinder assemblies, is that the manufacturing cost is substantially higher than a typical cast iron engine block with uncoated hypereutectic aluminum-silicon alloy pistons, and secondly, the plating processes are not environmentally friendly.
Contrary to the problems that arise when running hypereutectic aluminum-silicon alloy pistons and cylinders directly on each other, cast iron surfaces can be run directly on each other. The cast iron/cast iron mating surface combinations apparently are workable because the insoluble graphite in the structure provides a solid lubricant at the mating surface interface. Along the same line, U.S. Pat. No. 4,297,976 describes an engine in which uncoated hypereutectic aluminum silicon alloy pistons were run in cylinder bores composed of a hypereutectic aluminum-silicon alloy containing a solid lubricant of tin, lead and/or molybdenum. However, it is difficult to cast an engine block of a hypereutectic aluminum-silicon alloy with insoluble constituents, such as tin, lead or molybdenum, and have the insoluble particles uniformly distributed at the bore surface. Even if the alloy containing the solid lubricants was employed only as a cylinder liner as opposed to the entire engine block, there would be a casting problem, because the insoluble particles have a higher density than aluminum, and in a centrifugal casting process, which is the preferred manner of producing liners, the heavier insoluble constituents would migrate away from the inner diameter surface, where they are necessary in providing the solid lubricity at the mating surface, to the outer diameter surface where they have no value.